root/net/netfilter/xt_tcpmss.c

DEFINITIONS

1. tcpmss_mt
2. tcpmss_mt_init
3. tcpmss_mt_exit

/* Kernel module to match TCP MSS values. */

/* Copyright (C) 2000 Marc Boucher <marc@mbsi.ca>
 * Portions (C) 2005 by Harald Welte <laforge@netfilter.org>
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 2 as
 * published by the Free Software Foundation.
 */

#include <linux/module.h>
#include <linux/skbuff.h>
#include <net/tcp.h>

#include <linux/netfilter/xt_tcpmss.h>
#include <linux/netfilter/x_tables.h>

#include <linux/netfilter_ipv4/ip_tables.h>
#include <linux/netfilter_ipv6/ip6_tables.h>

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>");
MODULE_DESCRIPTION("Xtables: TCP MSS match");
MODULE_ALIAS("ipt_tcpmss");
MODULE_ALIAS("ip6t_tcpmss");

static bool
tcpmss_mt(const struct sk_buff *skb, const struct xt_match_param *par)
{
        const struct xt_tcpmss_match_info *info = par->matchinfo;
        const struct tcphdr *th;
        struct tcphdr _tcph;
        /* tcp.doff is only 4 bits, ie. max 15 * 4 bytes */
        const u_int8_t *op;
        u8 _opt[15 * 4 - sizeof(_tcph)];
        unsigned int i, optlen;

        /* If we don't have the whole header, drop packet. */
        th = skb_header_pointer(skb, par->thoff, sizeof(_tcph), &_tcph);
        if (th == NULL)
                goto dropit;

        /* Malformed. */
        if (th->doff*4 < sizeof(*th))
                goto dropit;

        optlen = th->doff*4 - sizeof(*th);
        if (!optlen)
                goto out;

        /* Truncated options. */
        op = skb_header_pointer(skb, par->thoff + sizeof(*th), optlen, _opt);
        if (op == NULL)
                goto dropit;

        for (i = 0; i < optlen; ) {
                if (op[i] == TCPOPT_MSS
                    && (optlen - i) >= TCPOLEN_MSS
                    && op[i+1] == TCPOLEN_MSS) {
                        u_int16_t mssval;

                        mssval = (op[i+2] << 8) | op[i+3];

                        return (mssval >= info->mss_min &&
                                mssval <= info->mss_max) ^ info->invert;
                }
                if (op[i] < 2)
                        i++;
                else
                        i += op[i+1] ? : 1;
        }
out:
        return info->invert;

dropit:
        *par->hotdrop = true;
        return false;
}

static struct xt_match tcpmss_mt_reg[] __read_mostly = {
        {
                .name           = "tcpmss",
                .family         = NFPROTO_IPV4,
                .match          = tcpmss_mt,
                .matchsize      = sizeof(struct xt_tcpmss_match_info),
                .proto          = IPPROTO_TCP,
                .me             = THIS_MODULE,
        },
        {
                .name           = "tcpmss",
                .family         = NFPROTO_IPV6,
                .match          = tcpmss_mt,
                .matchsize      = sizeof(struct xt_tcpmss_match_info),
                .proto          = IPPROTO_TCP,
                .me             = THIS_MODULE,
        },
};

static int __init tcpmss_mt_init(void)
{
        return xt_register_matches(tcpmss_mt_reg, ARRAY_SIZE(tcpmss_mt_reg));
}

static void __exit tcpmss_mt_exit(void)
{
        xt_unregister_matches(tcpmss_mt_reg, ARRAY_SIZE(tcpmss_mt_reg));
}

module_init(tcpmss_mt_init);
module_exit(tcpmss_mt_exit);